1. Field of the Invention
The present invention relates to a Li2O—Al2O—SiO2-based crystallized glass.
2. Related Art
A Li2O—Al2O3—SiO2-based crystallized glass containing 3-quartz and/or β-quartz solid solution has a low average expansion coefficient, and useful physical properties intrinsic to a crystallized glass based on this system, such as high rigidity and ultra-surface-smoothness after polishing. Because of these characteristics, there have been made a study of use of the crystallized glass based on this system as a mirror substrate material and a photomask substrate material of a next-generation semiconductor production device in which extreme ultraviolet lithography (EUVL) technology using extreme ultraviolet rays as a light source is employed. The crystallized glass required for these applications has been required to contain fewer remaining bubbles, which exert an influence on a surface shape, so as to have ultra-surface-smoothness.
Meanwhile, in the Li2O—Al2O3—SiO2-based crystallized glass, a precursor glass commonly has a high melting temperature of 1,450 to 1,600° C. in the production process thereof. In this crystallized glass, a clarifying agent has been added for the purpose of homogenizing and clarifying the glass melt during melting of the production process, and an arsenic component has frequently been used as the clarifying agent having the effect within the above-mentioned high temperature range. However, the arsenic component may exert an adverse influence on the human body and environment, and requirements of refraining from use of these components as much as possible have become higher and higher.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2011-173748    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2011-201763    Patent Documents 1 and 2 mentioned above disclose Li2O—Al2O3—SiO2-based crystallized glasses, and suggest some clarifying agents other than an As2O3 component and an Sb2O3 component.
In Patent Document 1, the clarification effect is exerted by inclusion of rare earth oxide and halogen when a Li2O—Al2O3—SiO2-based glass material is melted. However, halogen such as Cl and a compound containing halogen used in a glass material in Patent Document 1 are toxic, and apply a large burden on the human body and production facilities. The rare earth oxide used in Patent Document 1 is disadvantageous in view of stable supply and cost.
In Patent Document 2, the clarification effect is exerted by inclusion of SnO2 and additional clarifying agents such as Sb2O3, Cl−, Br−, and SO42− when a Li2O—Al2O3—SiO2-based glass material is melted. However, in a crystallized glass of Patent Document 2, SnO2 is used as the clarifying agent and there is a problem that desired physical properties cannot be obtained by devitrification of the crystallized glass. Among additional clarifying agents in Patent Document 2, Cl and Br, and a compound containing Cl and Br are toxic and apply a large burden on the human body and production facilities. In Patent Document 2, Sb2O3, which is a component capable of becoming environmentally harmful, is used as additional clarifying agents, thus being disadvantageous in view of an environmental burden.